The objectives of this study are a) to establish whether a circadian rhythm exists in uterine blood flow b) to establish whether a circadian rhythm exists in umbilical blood flow c) to examine the phase relationship between uterine blood flow and umbilical blood flow rhythms and d) to investigate the effect of environmental variables on the amplitude, phase and period of the rhythms. Chronically instrumented pregnant Dorset sheep of 129-131 days gestation are used. Under ketamine anesthesia electromagnetic flow transducers are applied to the dominant uterine artery and the common intra-abdominal umbilical vein. Catheters are placed in the maternal aorta, fetal aorta and amniotic cavity. These along with the transducer leads are exteriorized through a skin incision in the flank of the ewe. Experiments are performed 5 days postoperative to allow complete recovery from surgical procedure, and after a minimum of 3-7 days preoperative acclimation. Environmental conditions are stringently controlled.